Field of the Invention
This invention relates to a developing device, an image forming apparatus, and a control program of a developing device. More specifically, this invention relates to a developing device, an image forming apparatus, and a control program of a developing device, which can detect a rotational position of a developing roller, preventing complication of the device configuration.
Description of the Related Art
As electrophotographic image forming apparatuses, there are an MFP (Multi Function Peripheral) which has a scanner function, a facsimile function, a copying function, a function of a printer, a data transmitting function and a server function, a facsimile device, a copying machine, a printer, and so on.
An image forming apparatuses generally develop an electrostatic latent image formed on an image supporting body to form a toner image After transferring the toner image onto a sheet, a fixing device fixes the toner image on the sheet to form an image on the sheet. Some image forming apparatuses develop an electrostatic latent image on a surface of a photo conductor by using a developing device, to form a toner image, and transfer the toner image to a secondary transfer belt by using a primary transfer roller. The toner image on the secondary transfer belt is secondary transferred to a sheet by using a secondary transfer roller. In this instance, the photo conductor and the secondary transfer belt act as image supporting bodies.
In an electrophotographic process, density ununiformity (periodical ununiformity) may occur in the developing cycle in the sub scanning direction of the toner image, due to runout of an outer shape of the developing roller, when an electrostatic latent image is developed with toner by the developing roller of the developing device. An idea of a method for suppressing the density ununiformity is to adopt stricter screening criteria (inspection criteria) relating to runout of developing rollers at the time of manufacture of image forming apparatuses, to employ only image rollers of which the runout is small. However, since the method causes decreasing in yield of developing rollers, the method has a problem in that it causes increasing the cost of an image forming apparatus.
Another method for suppressing the periodical ununiformity above mentioned is to match phases of both a rotation period of a developing roller and a cycle of density ununiformity in the circumferential direction of the developing roller, wherein the density ununiformity is detected by an image density reading sensor. According to the method, the periodical ununiformity is reduced and suppressed. According to the method, a disk with slits is installed on the rotation shaft of the developing roller. The disk is detected by a photo sensor, such as a photo interrupter, or an encoder, so that the rotational position of the developing roller is detected.
The Document 1 below discloses a technique for detecting a rotational position of a developing roller by using a photo interrupter. According to the technique, a detection plate is directly connected to the rotation shaft of the developing roller. The detection plate rotates in conjunction with the developing roller. The detection plate has a slit. The photo interrupter detects the slit of the detection plate on every rotation of the detection plate, to detect the rotational position of the developing roller.
[Document 1] Japan Patent Publication No. 2000-98675
However, according to the another method above mentioned, a detector such as a photo interrupter or an encoder should be newly installed on a developing device. It has a problem which causes the complication of the device configuration and increase in the cost. Especially, from the viewpoint of the cost, the increase in a cost when a detector is newly installed is almost of the same as the increase in a cost when adopting stricter screening criteria relating to runout of developing rollers. Therefore, the another method above mentioned does not receive benefit by cost reduction.